From: "Walter Harley"
Subject: Re: TO-220 insulation
Date: Thu, 26 Dec 2002 19:00:01 -0800
Organization: Posted via Supernews, http://www.supernews.com
Reply-To: "Walter Harley"
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"Jordan" wrote in message
> I have to have two TO-220 transistors mounted on the same heatsink for
> pretty high-power job, and they will be dissipating quite a bit of heat.
> They must be insulated from each other. If I use mica insulators and
Mica and silicon goo have been used successfully for decades; no reason they
should fail for you. Yes, they do have a finite thermal resistance; no,
offhand I don't remember how much.
But nowadays you also have the option of using these cool little silicon
gaskets that have even lower thermal resistance, and don't make a big mess.
Digikey sells a bunch of kinds; see, for instance,
http://rocky.digikey.com/WebLib/Berquist/Sil-Pad%20400%20%20600.pdf for a
datasheet on one of them (randomly chosen).
Junction-to-case thermal resistance of TO-3 packages is generally
considerably less than TO-220. Conversely, some transistors are available
in fully-insulated TO-220-like cases, which could be directly mounted.
Check the case options for the transistors you need.
And remember that the heatsinks with lots of fins look impressive but do not
necessarily have lower thermal resistance than those with just a couple,
unless perhaps you're using forced air cooling.